Indicating that Musk's tweets may have been rushed, the spokesman for New York Mayor Bill de Blasio replied, also through Twitter, that "This is news for the City Council".

Most tech analysts believe Musk is not one to post ideas on Twitter without first fleshing out the idea.

Such a project would undoubtedly cost billions of dollars and require approvals, and likely funding, from federal, state and local officials to move forward.

Musk said that the Hyperloop's planned route would run from NY and include stops in Philadelphia, Baltimore and Washington.

Musk tweeted that there's still a lot of work needed to receive formal approval, but he's clearly used to the fast track.

In December 2016, Musk founded The Boring Company, which is dedicated to the tunneling model and moving beyond 2-D transportation infrastructure. One outcome could be to use the machines to construct tunnels for a transportation system known as Hyperloop, which would hurtle passengers through underground tubes in high-speed pods.

Musk mentioned last month that he had "promising conversations" with the mayor of Los Angeles for a similar tunnel structure in the city, and today Musk expanded that the first set of tunnels "are to alleviate greater LA urban congestion".

Contacted Thursday evening, Dunn told PhillyVoice "there are numerous hurdles for this unproven "Hyperloop" technology before it can become reality".

Musk did not elaborate on which government agency or official gave him the verbal approval.

Then, he admitted Friday morning, "Verbal approval was at Federal level". His new tunnel-boring firm, The Boring Company, would perform the work. "The groan-worthy joke that I make about tunnels is they have low expectations-there's nowhere to go but down".